A skeletal–smooth muscle interface mediates levator ani attachment to the rectum and vagina: a macroscopic and histological study

Abstract

Abstract We aimed to clarify the attachment architecture between the levator ani and the rectum and vagina using integrated macroscopic and histological analyses of human pelvic floor specimens. Macroscopic dissection consistently demonstrated whitish soft tissue at the interface between the levator ani and the lateral walls of the rectum and vagina. Histological and immunohistochemical analyses of the corresponding topographical regions revealed abundant smooth muscle components continuous with the outer muscular layers of each organ. In both organs, the smooth muscle extended beyond the organ walls, spread over the superior and inferior surfaces of the levator ani, and interdigitated between skeletal muscle bundles, forming a serrated (sawtooth-like) interface. No dense connective tissue structures resembling tendons or aponeuroses were identified, indicating that levator ani–visceral attachment does not conform to a classical tendon-mediated enthesis. Distinct attachment patterns were observed between the two organs. At the rectal interface, a subset of levator ani muscle fibers coursed toward smooth muscle continuous with the rectal longitudinal layer and formed an insertion-like arrangement. In contrast, at the vaginal interface, no insertion-like arrangement of levator ani muscle fibers into the vaginal wall was observed; instead, abundant vaginal smooth muscle extended laterally and enveloped adjacent levator ani muscle bundles. These findings provide a clearer anatomical characterization of a non-tendinous skeletal–smooth muscle interface between the levator ani and deformable pelvic visceral walls. This structural configuration may provide an anatomical basis for future studies examining how the levator ani interacts mechanically with these deformable pelvic organs. Mini-abstract : The levator ani attaches to the rectum and vagina through a non-tendinous skeletal–smooth muscle interface, rather than a classical tendon-mediated enthesis.